This invention relates to a tool for use in forming a lens and to a method of forming a lens and particularly but not exclusively to lenses used in eye glasses.
In a conventional laboratory for manufacturing eye glasses, complex curvature lenses are manufactured for assembly into the eye glasses. In most cases the lens is formed from a lens blank which has a front surface already shaped to a predetermined curvature. A lens of a particular prescription is then formed by cutting and polishing a rear surface of the lens to a calculated curvature related to the selected curvature of the front surface. Lenses such as bifocals and trifocals are usually formed by taking a front surface which has the bifocal or trifocal effect already formed on the front surface so that it is only necessary then to form the rear surface to the required curvatures.
In the conventional process, the lens is grasped by a system comprising applying to the front surface a covering layer of an adhesive film which acts to protect the front surface and at the same time to enable grasping of the front surface without damaging the front surface. Onto the film is cast a boss of a low melting point alloy so that the boss can be readily grasped thus holding the lens with the front surface remaining facing the boss and protected by the boss and the covering film and exposing the rear surface for the cutting action
In the first step of the cutting action, the lens is rough cut using a tool known as a Generator which is a milling tool in the form of a cutting ring, the axis of which is twisted to form the required radius of curvature and which then sweeps across the rear surface to provide the cutting action. This acts to reduce the lens to the required thickness and rough cuts the require curvatures which may differ in horizontal direction and vertical direction. These two curvatures are then formed by cutting in the two separate directions so that the lens is held stationary while the tool rotates and sweeps across the rear surface in the cutting action. The Generator is a machine well known to one skilled in the art. One example is manufactured by Optical Works Machinery of Muskogee, Okla. and has been available for many years so that the full details constitute well known prior art.
The rough cut lens in then moved to a polishing and finishing process in which a tool is applied to the rear surface and moved in a figure of eight polishing action relative to the rear surface to smooth the rear surface to the required level of finish. The tool thus has a surface very accurately contoured to the required shape of the finished lens so the polishing action causes the rear surface of the lens to accord to the surface of the tool. A machine of this type is well known to one skilled in the art and is known as a "Cylinder Machine" one example of which is manufactured by Optical Works Machinery of Muskogee, Okla.
Conventionally these machines have tools which are manufactured in a highly complex and expensive process in which the tool is initially rough cast in a suitable metal such as aluminum or steel and then is finished using a time consuming and expensive milling process so that the required level of surface finish is obtained. It will be appreciated that the finish required must be very highly accurate and very highly polished since this is the finish that would be applied to the lens when the lens is polished to accord to this surface.
In previous years the lens was applied directly to the contoured surface of the tool and a suitable slurry or liquid applied between the lens and the surface for the polishing or grinding action.
In more recent years a thin layer of a polishing material as an integral thin film is adhesively applied to the surface of the tool with the layer being sufficiently thin to follow the surface of the tool. The tool can then be used with a number of different layers of different roughness to provide an initial grinding action followed by a finer polishing action and finally a very fine finishing action.
As the tool must accord directly to the required shape of the rear surface of the lens, it is necessary for a laboratory to carry a very large number of such tools. In view of the complex manufacturing technique for the tools including the initial rough casting followed by the skilled machining necessary, the cost of the inventory of such tools is very high so that only a relatively limited number of laboratories are set up for manufacture of the eye glasses.
In recent years however there has been a move toward the manufacture of eye glasses on a retail site so that the customer can be supplied with eye glasses within a very short period of time for example one hour. However the cost of the necessary tools and the inventory of the tools is very high and seriously detracts from the economics of the on-site manufacturing process. Up till now the manufacture of the tool from a rough cast metal followed by the skilled machining process has been considered to be essential in view of the requirement for the very accurate exact contour of the tool surface and in view of the requirement for vigorous clamping action of the tool to ensure that it is held fixed while the relative movement between the lens and the tool is carried out.